仪器网（yiqi.com）欢迎您！

参与评论

评论

登录后参与评论

全部评论(2条)

• 

  初心暖哭i 2017-08-14 00:00:00

  A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch， but other operating principles are also used， such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal， or where several circuits must be controlled by one signal.

  赞(11)

  回复(0)

  评论

  评论

  登录后参与评论

• 

  sfppscy72557 2013-11-05 00:00:00

  Time relay is used in all sorts of protection and automatic device as back-up component， make component be accusinged achieves the delay time of a need， in protector in order to comes true advocate of protection and back-up protection cooperate optionally. Intelligent time relay is to develop the new generation that rise to control apparatus in recent years， it uses 8 microcomputer to be core chip， for relay of type of face plate of electrify delay time. On， canal of below two numbers shows， used distinctive interference rejection technology， can apply extensively at metallurgy， electron， mechanical， spin， plastic refrigeration， electric stove oven need time pilot situation Frame Relay is a WAN protocol that has enjoyed a wide implementation since the late 1980s. Over the next few weeks， I will be writing newsletters that concentrate on Frame Relay topics as they pertain to the CCNA/CCNP exams. This week's article offers a basic introduction to Frame Relay. Frame Relay is a WAN protocol that has seen wide deployment since 1991. Frame Relay has been described as a streamlined version of the ancient X.25 protocol. Frame Relay (Frame) was originally designed to be used with ISDN， and operates on the physical and data link layers of the OSI model. Frame standards are maintained by the Frame Relay Forum， which was originally comprised of Cisco， DEC (Digital Equipment Corporation)， Northern Telecom， and Stratacom. Frame is a packet-switched protocol， so Frame does not require expensive leased lines to operate. Frame uses either a PVC (permanent virtual circuit) or a SVC (switched virtual circuit) to provide a data connection. As a packet-switched protocol， Frame supports variable packet sizes and only uses bandwidth when it has data to send. Frame traffic is routed to the correct destination using an address called a DLCI (Data-Link Connection Identifier). The DLCI is a 10-bit number that is found in the Frame header， and is used to identify the virtual circuit (VC). The DLCI is only significant to the local network (a service provider may have several customers that use the same DLCI on their networks). Equipment on a Frame network can be divided into two categories: DTE (data terminal equipment)， and DCE (data communications equipment). DTE equipment is usually customer premises equipment such as workstations， servers， or routers. DCE equipment provides the clocking and switching for the Frame circuit. In most cases， the DCE equipment is a packet-switch owned by the service provider. Frame Relay has several integrated congestion control mechanisms. FECN (forward-explicit congestion notification) and BECN (backwards-explicit congestion notification) are both single bits in the Frame header. If congestion is detected on a virtual circuit， the DCE device sets the FECN bit if the congestion is detected from source to destination， and the BECN bit if the congestion is for the opposite direction. The DE (discard eligibility) bit indicates a packet has lower importance when set. If congestion occurs， packets with the DE bit set are dropped. That concludes the basic introduction to Frame Relay. Next week， I will be writing about extensions to the original Frame specification， and configuring Frame on Cisco routers. Until then， have a great week.

  赞(15)

  回复(0)

  评论

  评论

  登录后参与评论